1. Field of the Invention
The present invention relates to an antenna device, having a circumferentially variable size, typically embedded in a wristband, for use with a radio which is generally worn on the arm of a person. More particularly, the antenna device of the present invention provides a method and apparatus for automatically compensating for changes in antenna gain and resonance frequency which result from changes in the antenna size.
2. Description of the Related Art
FIG. 34 shows an example of a conventional approach to the construction of portable radio transmitters or receivers. More particularly it shows a proposed antenna device for an arm-attached type radio apparatus capable of being carried while worn on a wrist. FIG. 34 shows an arm-attached type radio apparatus 90 that includes a case body 92 (the main body of the radio apparatus) accommodating a circuit board for the radio apparatus; and an arm attaching band 91 connected to both sides of case body 92. Attaching band 91 has first and second band members 91a, 91b formed of an insulating material. First and second band members 91a, 91b include, respectively, first and second band-like conductor plates 93a, 93b that are embedded within the band members. First and second conductor plates 93a, 93b, are electrically coupled to the radio circuit, and are electrically coupled at their free ends to band connector portions 91c, 91d of first and second band members 91a, 91b. When first and second band members 91a, 91b are connected by way of band connector portions 91c, 91d, first and second conductor plates 93a, 93b form a loop through circuit 94, thus forming an antenna 95 as shown in FIG. 35. In radio apparatus circuit 94, a high-frequency amplifier circuit 94b is coupled via coupling capacitor 94a to first conductor plate 93a, and a variable capacitor 94c is connected between first conductor plate 93a and ground. Note that the side of second conductor plate 93b is fixed at ground potential.
However, because the thickness of a wearer's wrist varies, there is a problem with antenna 95. That is, depending on the connecting position of first and second band members 91a, 9lb, the circumference thereof varies and consequently, the antenna inductance value changes, and antenna gain is lowered. In other words, because, the inductance of antenna 95 is changed as the band size is changed by the wearer, the resonance frequency is shifted and antenna gain is lowered.
In view of the above problems, an object of the present invention is to achieve an antenna device for arm-attached type radios capable of obtaining a stable gain without being affected by the difference in the band size of the wearer.